1. Field of the Invention
The invention relates to aluminum based alloys having improved strength at elevated temperatures through the addition of rare earth elements, and to powder products produced from such alloys. More particularly, the invention relates to Al-Fe-Si-X-RE alloys (RE signifies rare earth elements) that have been rapidly solidified from the melt and thermomechanically processed into structural components having improved elevated temperature strength.
2. Brief Description of the Prior Art
Methods of obtaining improved tensile strength in aluminum based alloys have been taught by U.S. Pat. No. 2,963,780 to Lyle et al.; U.S. Pat. Nos. 2,967,351and 3,462,248 to Roberts et al.; and U.S. Pat. Nos. 4,828,632, 4,878,967 and 4,879,095 to Adam et al. However, these teachings alloys propose increasing quantities of transition element and/or higher cooling rates during casting of the alloys for the elevated temperature strength thereof to be increased. It would be desirable if rare earth elements could be added to rapidly cooled alloys containing transition metal elements to improve the elevated temperature strength without the necessity of forming further intermetallics or increasing the quench rate. Yet, prior art workers have heretofore not pursued this course.
The addition of rare earths to aluminum has been attempted by U.S. Pat. No. 4,379,719 to Hilderman et al., where rapidly quenched aluminum alloy powder contains 4 to 12 wt% iron and 1 to 7 wt% cerium or other rare earth metals from the lanthanum series. Other examples of rare earth additions include: A.K. Gogia et al.; J. of
Mat. Science, 20, pp. 3091-3100 (1985); S.J. Savage et al.; Processing of Structural Metals by Rapid Solidification, Conf. Proc. ASM Materials Week '86 Orlando, FL, Ed. F.H. Froes and S.J. Savage, ASM International, pp. 351-356 (1986); Y.R. Mahajan et al., J. of Mat. Science, 22, pp. 202-206 (1987); A. Ruder et al., J. of Mat. Science, 25, pp. 3541-3545 (1990) and C.S. Sivaramakrishnan et al., J. of Mat. Science, 26, pp. 4369-4374 (1991). However, these rare earth additions are integral in the formation of the strengthening intermetallics having general composition Al.sub.x Fe.sub.y Re.sub.z (where Re refers to the rare earth).
There remains a need in the art for rapidly solidified aluminum base alloys having improved elevated temperature strengths.
3. Summary of the Invention
The present invention provides rapidly solidified aluminum base alloys wherein elevated temperature strengths are markedly improved without the necessity of increasing the volume fraction of intermetallics therewithin. Generally stated, the aluminum based alloy of the invention consists essentially of the formula Al.sub.bal Fe.sub.a M.sub.b Si.sub.c R.sub.d, wherein M is at least one element selected from the group consisting of V, Mo, Cr, Mn, Nb, Ta, and W; R is at least one element selected from the group consisting of La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb, and Y, "a" ranges from 3.0 to 7.1 atom %; "b" ranges from 0.25 to 1.25 atom %; "c" ranges from 1.0 to 3.0 atom %; "d" ranges from 0.02 to 0.3 atom % and the balance is aluminum plus incidental impurities, with the provisos that (i) the ratio [Fe+M]:Si ranges from about 2.0:1 to 5.0:1 and (ii) the ratio Fe:M ranges from about 16:1 to 5:1.
To provide the desired levels of ductility, toughness and strength needed for commercially useful applications, the alloys of the invention are subject to rapid solidification processing, which modifies the alloy's microstructure. The rapid solidification processing method is one wherein the alloys are placed into the molten state and then cooled at a quench rate of at least about 10.sup.5 .degree.Cs.sup.-1 and preferably about 10.sup.5 to 10.sup.7 .degree.Cs.sup.-1 to form a solid substance. More preferably this method should cool the molten metal at a rate greater than about 10.sup.6 .degree.Cs.sup.-1 i.e. via melt spinning, splat cooling or planar flow casting which forms a solid ribbon or sheet. These alloys have an as cast microstructure which varies from a microeutectic to a microcellular structure, depending on the specific alloy chemistry. In alloys of the invention the relative proportion of these structures is not critical.
Consolidated articles of the invention are produced by compacting particles composed of an aluminum based alloy consisting essentially of the formula Al.sub.bal Fe.sub.a M.sub.b Si.sub.c R.sub.d, wherein M is at least one element selected from the group consisting of V, Mo, Cr, Mn, Nb, Ta and W; R is at least one element selected from the group consisting of La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb and Y; "a" ranges from 3.0 to 7.1 atom %; "b" ranges from 0.25 to 1.25 atom %; "c" ranges from 1.0 to 3.0 atom %; "d" ranges from 0.02 to 0.3 atom % and the balance is aluminum plus incidental impurities, with the provisos that (i) ratio [Fe+M]:Si ranges from about 2.0:1 to 5.0:1 and (ii) the ratio Fe:M ranges from about 16:1 to 5:1. The particles are heated in a vacuum during the compacting step to a pressing temperature ranging from about 300.degree. C. to 500.degree. C., which minimizes coarsening of the dispersed intermetallic phases. Alternatively, the particles are put in a can which is then evacuated, heated to between 300.degree. C. and 500.degree. C. and then sealed. The sealed can is heated to between 300.degree. C. and 500.degree. C. in ambient atmosphere and compacted. The compacted article is further consolidated by conventional methods such as extrusion, rolling or forging.
The consolidated article is composed of an aluminum solid solution phase containing a substantially uniform distribution of dispersed intermetallic phase precipitates of approximate composition Al.sub.13 (Fe,M).sub.3 Si. These dispersoids are fine intermetallics measuring less than 100 nm in all linear dimensions thereof. Alloys of the invention, containing these fine dispersed intermetallics are capable of withstanding the pressures and temperatures associated with conventional consolidation and forming techniques such as forging, rolling and extrusion without substantial growth or coarsening of these intermetallics that would otherwise reduce the strength and ductility of the consolidated article to unacceptably low levels. The rare earth elements added to the alloys of the invention do not form any new intermetallic phases therein; but instead substantially stay in solid solution of the aluminum matrix phase. At elevated temperatures in excess of approximately 260.degree. C. the action of the rare earth elements in the aluminum solid solution is to impede the motion of dislocations around the dispersed intermetallic phase through the retardation of the climb process necessary for these dislocations to circumvent the dispersed intermetallic phase therein. This retardation process causes a marked increase in strength of the material at these elevated temperatures, such strength increase ranges from about 5 to 15 percent.
Advantageously, the improved elevated temperature strength of articles produced in accordance with the invention makes such articles especially suited for use in gas turbine engines, missiles, airframes, landing wheels, and the like.